"CHLORITIZED" VERMICULITE IN A KOREAN ULTISOL STUDIED BY ULTRAMICROTOMY AND TRANSMISSION ELECTRON MICROSCOPY

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1 Clays and Clay Minerals, Vol. 37, No. 3, , "CHLORITIZED" VERMICULITE IN A KOREAN ULTISOL STUDIED BY ULTRAMICROTOMY AND TRANSMISSION ELECTRON MICROSCOPY KOJI WADA AND YASUKO KAKUTO Faculty of Agriculture, Kyushu University 46, Fukuoka 812, Japan Abstract--"Chloritized" vermiculite coexisting with kaolin-group minerals, vermiculite, mica, and gibbsite in the /~m fraction separated from a Korean Ultisol was studied by ultramicrotomy and highresolution electron microscopy. The (001) lattice images observed from thin sections showed that the "chloritized" vermiculite was composed of particles which differed in their sha, pe and the stacking of silicate layers. On saturation with K +, the silicate layers had spacings of - A, with -12-/~ layers being dominant. The 8-7-A layers of a few particles appeared to be segregated close to the basal plane surfaces, but not necessarily close to the edge surfaces. The silicate layers were more or less curved and wavy and were commonly discontinuous. They formed packets (0-500,~ thick) by stacking, which were associated "face-to-face" and/or "edge-to-edge" or stacked l~arallel or subparallel to form particles of different shapes. The extent of interlayer collapse of the -A layers on heating differed from one particle to the other, but was fairly uniform within a particle. Treatment with hot Va M sodium citrate dissolved the interlayer material and possibly the material present between the packets from the "chloritized" vermiculite and from a few fine plates of kaolin-group minerals. The interlayer material and its chemical composition, therefore, could not be determined. ~0~g~&r_~5~o ~:, ~-4"~INNla~s~&,y b 0oo t~b 500 I o~g~) ~NNb~N ~, --'Do)t~.~P~'c~gt~t~O--~"c~o~Co ~ 1/3 M ~-r 2.,~d-b U,)~t.~J:.,~l.g, " ~ Key Words--Chloritized vermiculite, Stacking arrangement, Transmission electron microscopy, Ultramicrotomy, Vermiculite. INTRODUCTION Many soils under leaching conditions in temperate regions contain partly interlayered or "chloritized" vermiculites as major clay mineral species (Rich, 1968; Barnhisel, 1977; Douglas, 1977). Like vermiculite, "chloritized" vermiculite gives a -~ reflection on MgZ+-saturation and air-drying, but the shift of this -~ reflection to /k on K + saturation is incomplete even after heating and occurs only after extraction of "interlayer material" by a hot '/3 M sodium citrate treatment. Inasmuch as A1 is extracted by the treatment, "chloritized" vermiculite has been considered as vermiculite in which the interlayer space is partly filled with polymer hydroxy-a1 cations. On the other hand, studies by Wada and Kakuto (1983a, 1983b) of"chloritized" vermiculites in several Copyright , The Clay Minerals Society 263 red and yellow soils (Ultisols and Alfisols) in Korea showed that (1) the "interlayer material" extracted by the sodium citrate treatment contained not only AI but Si, (2) its molar Si/A1 ratio was close to 1.0, and (3) its infrared spectra had features of kaolin-group minerals. Based on these and other analyses, the term intergradient vermiculite-kaolin mineral, in which each -/~ layer laterally transforms to two 7-~ layers, was proposed for "chloritized" vermiculite. Douglas (1977) pointed out that vermiculite in acid soils invariably has a hydroxy-a1 interlayer, yet virtually no literature exists that describes the actual structure and composition of soil vermiculite. This situation does not seem to have changed since Douglas' report. "Chloritized" vermiculite has been recognized only as clay-size particles, too small for single crystal X-ray diffraction, and only in mixtures with other clay min-

2 264 Wada and Kakuto Clays and Clay Minerals erals. In the present study, ultramicrotomy and highresolution transmission electron microscopy (HRTEM) were used to observe stacking of silicate layers in individual clay particles of an Ultisol, in which "chloritized" vermiculite was identified as a major clay mineral. Clay samples MATERIALS AND METHODS The soil selected for the study was the IIB23t(g) ho -~ rizon of a Typic Hapludult (Wada and Kakuto, 1983a), which is a heavy clay derived from old alluvium associated with granite and gneiss. The soil shows an acid reaction (ph (H20) = 5.4; ph (1 M KC1) = 3.80). Clay fractions (<0.2 and um) were separated from the soil by repeating dispersion at ph (NaOH) with sonic vibration and sedimentation. Dissolution treatments and X-ray powder diffraction analysis Treatments with dithionite-citrate (Mehra and Jackson, 1960) and with 1/3 M sodium citrate at 0~ (Tamura, 1958; Wada and Kakuto, 1983a) were used, respectively, to remove iron oxide and "interlayer material" from "chloritized" vermiculite. The treated clays were saturated with Mg 2 and K and oriented on glass slides. They were analyzed by X-ray powder diffraction (XRD) after air-drying or heating at 60 ~ 300 ~ and 550"C for 1 hr with a Rigaku Geigerflex (CoKa, 30 kv, and ma). Ultrarnicrotomy and HRTEM The #m fractions treated differently and saturated with K (30-60 mg) were washed successively by centrifugation with 5-ml portions of water and 70%, 85%, and 0% ethanol, oxypropyrene, and an oxypropyrene-epoxy resin mixture. Each washed sample was then suspended in 1.5 ml of the epoxy resin plus its hardening reagent. A layer of this suspension, about 1 mm thick, was cast in a fiat-bottom plastic vessel, evacuated, and cured at 40 ~ 50 ~ and 60~ each for 1 day. A specimen block of appropriate size was removed and trimmed with a razor blade on the specimen holder of LKB 2088V ultramicrotome. The trimmed block was sectioned at a thickness of /~ using a diamond knife, and the thin sections were collected on a plastic microgrid. Clay particles sectioned perpendicular to the stacking of silicate layers were examined with a JEOL 0B electron microscope, operating at 0 kv. RESULTS AND DISCUSSION XRD analyses of the t~m fractions before and after the sodium citrate treatment showed that the major mineral component was "chloritized" vermiculite, with lesser kaolin-group minerals, vermiculite, interstratified vermiculite/"chloritized" vermiculite, 72 Mg-~ K -6~ "v K pro A A B 0 K ~ 60 "C K- 72,r 0.2 pm 72 i4a 1~ 1'o ~ 1" 1" 1'~ lb ~ ~2e, CoKg radiation Figure 1. X-ray powder diffraction patterns of parallel oriented clays treated with dithionite-citrate (A) and then with 1/3 M sodium citrate (B). mica (Figure 1), and gibbsite. The d-values of the 001 reflection used for identification of the layer silicates are shown in Table 1. "Chloritized" vermiculite was also present in the <0.2-~tm fraction, but its content was less than that in the #m fraction, and it collapsed more readily on drying and heating. Lowmagnification electron micrographs showed that the ~tm fraction contained not only particles > 0.2 um, but many thin plates <0.2 um in size (Figure 2a). The thin sections showed that some fine plates coexisted with the large particles on their surfaces, but some were also oriented with each other "face-to-face" and/ or "edge-to-edge" to form the larger particles (Figure 3). Mineral species constituting each particle observed by HRTEM were identified by measuring the spacings of the (001) lattice images that were generated by the thin section and by referring to the XRD patterns of the minerals in the and <0.2-#m fractions. Most of the large particles were identified as "chloritized" vermiculite. They occurred as thin to thick, more or less curved plates (Figures 3a, 3c, 3d, and 4), twisted or crumpled plates (Figures 3b, 5, and 6), and lenseor spindle-shaped particles (Figures 3e, 7, and 8). In all of these particles the silicate layers, as indicated by the lattice images, were more or less curved or wavy and the arrangement of these layers was neither uniform nor continuous throughout the particle. On the other hand, the silicate layers tended to form packets by stacking. The packets were as thick as 500/k, but most were A thick. The isolated packets may correspond to the fine plates described above (Figure 3). The packets were either associated "face-to-face" and/or "edge-to-edge" or were stacked parallel or subparallel, forming the particles of different shapes. Nonimaged portions between the packets were also noted within the particle (Figures 4-7), suggesting the pres-

3 Vol. 37, No. 3, 1989 "Chloritized" vermiculite 265 Figure 2. Electron micrographs of the #m fraction; (a) treated with dithionite-citrate and (b) then treated with I/3M sodium citrate. Marker = 0.5 urn. ence of poorly crystalline material or the distortion of the packets with respect to the electron beam. "Chloritized" vermiculite particles were constructed of silicate layers having spacings of - ~ and 8-7 ]k (Figures 4-8), the -12-Zk layers being dominant. These silicate layers were c o m m o n in K+-saturated and mr-dried clays containing "chloritized" vermiculite by X R D (Figure 1; also see, e.g., Barnhisel, 1977), but layers other than the -~ layer were not assigned to "chloritized" vermiculite. The packets were formed by a stacking of the layers having the same spacings (Figures 4-6) and by a random interstratification of the layers having different spacings (Figures 7 and 8). The 7-8-Zk layers were noted in a few particles, commonly forming a packet that segregated in a portion close to the basal plane surfaces but not necessarily close to the edge surfaces (Figure 8). As described in Introduction, the presence of kaolingroup mineral as "interlayer material" in "chloritized" vermiculite was inferred by Wada and Kakuto (1983a). A partial, lateral transformation of each -~ layer to two 7-/k layers, one complete and one incomplete and inverted to each other, was proposed to explain the obtained data. Such 7 - ~ layers were, however, not detected in the present H R T E M study. No particular indication was found for lateral transformation of a -~ layer to a layer or layers having spacings other than A. Table 1. The d-values (fit) of the 00l reflection used for identification of layer silicates. Tremmem j Mineral "Chloritized" vermiculite Kaolin-group mineral Vermiculite Interstratified vermiculite/ "chloritized" vermiculite Mica t See text for details. M g "12 K--60~ K ~ K "(2 ~ 7.2 ~ ~ l > not observed ~ 12~ ->

4 266 Wada and Kakuto Clays and Clay Minerals Figure 5. Electron micrograph of thin section of "chloritized" vermiculite showing twisted arrangement of silicate layers. Marker = 500/~. Figure 3. Electron micrographs of thin sections of "chloritized" vermiculite particles of different shapes. Marker = 0.5 /zm. A few large plates consisted of a uniform stacking o f straight -~ or nearly straight -~ layers. The plates constructed from the -~ layers were identified as vermiculite and those from the -,~ layers as chlorite, because the plates similar to the latter and constructed from the -~ layers alone were found in the sample heated at 300~ Figure 9 shows a vermiculite plate constructed from straight -/~ layers in its core, but Figure 4. Electron micrograph of thin section of "chloritized" vermiculite showing curved arrangement of silicate layers. Marker = 500 A. showing possibly transformation from vermiculite to "chloritized" vermiculite and a kaolin-group mineral at its periphery. Discrete kaolin particles were noted largely as fine plates (Figure 8), as indicated by the X R D pattern o f the <0.2-#m fraction (Figure 1). Large kaolin particles were also noted, but they were fewer than expected from the intensity o f the 7-/~ X R D reflection of the #m fraction. A c o m m o n coexistence o f a kaolin-group mineral and "chloritized" vermiculite and the existence o f A layers in some particles o f "chloritized" vermiculite (Figures 8 and 9) suggest a close genetic relation between the two minerals. X R D analysis showed that K+-saturation and heating at 300~ resulted in a partial interlayer collapse of "chloritized" vermiculite (Figure 1). H R T E M s o f the corresponding thin sections showed that the layer spacings in "chloritized" vermiculite decreased uniformly in each particle to /~. The interlayer collapse tended to be greater in particles having regular stacking o f straight layers (Figure a) than in particles having less regular stacking of curved layers (Figure b). As shown in the X R D patterns (Figure 1), the sodium Figure 6. Electron micrograph of thin section of "chloritized" vermiculite showing crumpled arrangement of silicate layers. Marker = 500 A.

5 Vol. 37, No. 3, 1989 "Chloritized" vermiculite 267 Figure 9. Electron micrograph of thin section of vermiculite showing possible transformation to "chloritized" vermiculite and kaolin-group mineral. Marker = 500 A. citrate treatment resulted in the removal of the interlayer material from "chloritized" vermiculite. The lowmagnification electron micrographs suggest the thinning of particles as a whole and the dissolution of some finer plates (Figures 2a and 2b). H R T E M s of the thin sections showed that "chloritized" vermiculite from which the "interlayer material" was r e m o v e d had layer spacings larger and more variable (Figures 1 1 and 12) than those indicated by X R D analysis (Figure 1). These data suggest that the interlayer collapse on K+-saturation was incomplete in the procedure adopted for embedding in the epoxy resin. The silicate layers, however, became straighter and more extended, and their tendency to form packets and associated non-imaged portions was less noticeable (Figures 11 and 12). On the other hand, the complex interstratification of the layers having different spacings and the portions composed of 8-7-Zk layers as well as large kaolin particles remained after the treatment (Figure 12). The sodium citrate treatment of the #m fraction dissolved interlayer material and possibly material present between the packets from "chloritized" vermiculite and from a few of the fine plates of kaolingroup minerals. The dissolution of the kaolin-group mineral was also indicated by earlier chemical analysis and difference infrared spectroscopy (Wada and Kakuto, 1983a). Si (9.5% SiO2) and A1 (7.8% A1203) were dissolved by the treatment from the #m fraction of the same soil. The molar Si/A1 ratio of the extracted material was about 1.0. I f the dissolution of the kaolingroup mineral and the hydroxy-a1 interlayer in "chloritized" vermiculite took place simultaneously, the Si/ A1 ratio of the extracted material should have been < 1.0, unless the a m o u n t of A1 present as hydroxy-a1 interlayer was insignificantly small. The presence of interlayer AI (6.3% Al:O3, Barnhisel, 1977; 13.6% A1203, Frink, 1965), was, however, estimated in clays Figure 8. Electron micrographs of thin sections of "chloritized" vermiculite showing segregation of 8-7-A layers and kaolin-group mineral. Marker = 500/~. Figure, Electron micrographs of thin section of "'chloritized" vermiculites heated at 300~ which differ in regularity of stacking of silicate layers. Marker = 500/k. Figure 7. Electron micrograph of spindle-shaped thin section of"chloritized" vermiculite. (a) Whole particle. (b) Stacking of silicate layers in a packet. Marker = 500 A.

6 268 Wada and Kakuto Clays and Clay Minerals ACKNOWLEDGMENTS The authors thank the Ministry of Education, Science and Culture of Japan for Grant-in- Aid for Scientific Research (No ). Figure 11. Electron micrographs of thin section of "chloritized" vermiculite treated with 1/3 M sodium citrate. Marker = 500 A. in which the cation-exchange capacity of 40 meq/1 O0 g was reduced by blocking due to hydroxy-al. The interlayer material of studied "chloritized" vermiculite and its chemical composition still remain open to question. REFERENCES Barnhisel, R. I. (1977) Chlorites and hydroxy interlayered vermiculite and smectite: in Minerals in SoilEnvironments, J. B. Dixon and S. B. Weed, eds., Soil Science Society of America, Madison, Wisconsin, Douglas, L.A. (1977) Vermiculites: in Minerals in SoilEnvironments, J. B. Dixon and S. B. Weed, eds., Soil Science Society of America, Madison, Wisconsin, Frink, C. R. (1965) Characteristics of aluminum interlayers in soil clays: Soil Sci. Soc. Amer. Proc. 29, Mehra, O. P. and Jackson, M. L. (1960) Iron oxide removal from soils and clays by a dithionite-citrate system buffered with sodium bicarbonate: in Clays and Clay Minerals, Proc. 8th Natl. Conf., Norman, Oklahoma, 1959, Ada Swineford, ed., Pergamon Press, New York, Rich, C. I. (1968) Hydroxy interlayers in expansible layer silicates: Clays & Clay Minerals 16, Tamura, T. (1958) Identification of clay minerals from acid soils: J. Soil Sci. 9, 1-7. Wada, K. and Kakuto, Y. (1983a) Intergradient vermiculitekaolin minerals in a Korean Ultisol: Clays & Clay Minerals 31, Wada, K. and Kakuto, Y, (1983b) A new intergradient vermiculite-kaolin mineral in 2:1 to 1:1 mineral transformation: in P~trologie des Alterations et des Sols, Science G~ologiques M~moire 73, D. Nahon and Y. Noack, eds., Universit6 Louis Pasteur de Strasbourg, Strasbourg, France, (Received 30 September 1988; accepted 25 November 1988; Ms. 1838) Figure 12. Electron micrographs of thin sections of "chlofitized" vermiculite and kaolin-group mineral treated with 1/3 M sodium citrate. Marker = 500 A.